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NB 3 NeuroPhys > Diseases of the Neuromuscular Junction and Motor Unit > Flashcards

Diseases of the Neuromuscular Junction and Motor Unit Flashcards

1
Q

describe lesions of the soma

A
  • diseases
    • ALS
    • poliomyelitis
  • manifestations
    • muscle atrophy and weakness
    • hyporeflexia
    • fasciculations and fibrillations
    • eventual loss of muscle fibers (replaced by non-contractile fibrous CT)
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2
Q

describe poliomyelitis

A
  • viral infection primarily attacking ventral spinal grey matter
  • etiology:
    • small RNA viruses of Enterovirus genus
    • transmission is via fecal-oral route and aerosol droplets
  • clinical features
    • profound asymmetrical muscle weakness
    • affected limbs become floppy and poorly controlled - acute flaccid paralysis (AFP)
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3
Q

describe clinical features of poliomyelitis

A
  • destroyed motor neurons do not regenerate and denervated uscles will be unable to contract
  • extesive paralysis of the trunk may occur
  • mid-cervical involvement (C3,4,5) can paralyze diaphragm, necessitating ventilation
    • the tank respirator (iron lung) has some advantages over positive pressure applied through a tracheostomy and is still used occasionally
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4
Q

describe diseases related to lesions of Schwann cells

A
  • Guillain-Barre
  • diphtheria
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5
Q

describe the effect of botulinum toxin

A
  • botulinum toxin
    • toxic anerobic bacterial protease that reduces ACh release by acting on presynaptic exocytotic proteins
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6
Q

describe the mechanism of action of alpha-latrotoxin

A
  • toxin in venom of the black widow spider that causes massive release of ACh
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7
Q

describe the mechanism of action of beta-bungarotoxin

A
  • toxin in venom of snake initially provokes ACh release followed by ACh depletion by acting on proteins in nerve terminals involved in exocytosis
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8
Q

describe the mechanism of action of curare (delta-tubocurarine)

A
  • blocks nAChRs
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9
Q

describe Lambert-Eaton syndrome

A
  • this is a presynaptic disorder of neuromuscular transmission in which release of ACh is affected
  • strength increases with sustained or repeated muscular contraction in otherwise weakened patients because of the increase in concentration of synaptic ACh
  • etiology:
    • autoimmune disease where antibodies attack presynaptic voltage-gated Ca channels
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10
Q

describe the physiological changes at the end plate in Lambert-Eaton syndrome

A
  • amplitude of miniEPP is unchanged
  • reduced amplitude of EPP
    • reduced quantal content
  • many EPPs do not attain threshold in muscle fibers
  • waxing response in EMG (relates to facilitating neuromuscular block)
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11
Q

describe the EMG

A
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12
Q

describe the treatment of Lambert-Eaton syndrome

A
  • removal of underyling tumor and immunosuppressive drugs
  • plasma exchange
  • calcium gluconate to enhance Ca influx into nerve terminal
  • 4-aminopyridine (potassium channel blocker) to prolong presynaptic action potential and improve transmitter release
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13
Q

describe the deficiency of ACh-esterase can lead to myasthenia

A
  • deficiency of ACh-esterase in synaptic cleft
    • EPP amplitude is larger and longer than normal
    • single motor nerve stimuli delivered at low freq. cause single muscle twitches
    • high-freq. motor nerve stimuli produce temporal summations of EPPs and cause depolarization block of muscle
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14
Q

describe how slow channel syndrome can lead to myasthenia

A
  • ACh binding to nAChRs causes prolonged opening of ACh receptor channels and consequently, depolarization block
  • inherited rare-condition presents at birth or early childhood
    • muscle weakness
    • rapid fatigue
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15
Q

describe myasthenia gravis

A
  • chronic autoimmune diseases with antibodies to nAChRs develop and reduce transmission at the NMJ
  • symptoms:
    • weakness of somatic muscles
      • not associated with denervation
    • fatigability
    • temporary restoration of strength after rest
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16
Q

describe the interaction of antibody and nAChR

A
  • antibodies bind to alpha subunits of nAChR
  • antibodies do not compete with ACh for binding sites on subunits
  • antibodies cross-link neighboring nAChRs
    • endocytosis of nAChRs in endplate membranes –> lysosomal destruction
    • enhanced removal rate of nAChRs but no enhanced rate of insertion of nAChRs into end plate membrane
    • fall of density of nAChRs at end plate
17
Q
A
18
Q

describe the diagnosis and treatment of MG

A
  • diagnosis
    • repeated clenching/unclenching of fist -> weakness
    • Tensilon test (edrophonium), which is a ACh-esterase inhibitor, so muscular strength recovers temporarily
  • treatment
    • immunosuppressing medication: azathioprine, corticosteroids
    • pyridostigmine (AChE inhibitor)
19
Q

describe lesion of the chloride channel (myotonia congenita)

A
  • autosomal dominant disease, influencing the gene encoding Cl channels in the muscle membrane
    • leads to a small number of chloride channels in muscle
    • in normal muscles, the numerous Cl channels help to keep the membrane potential close to ECl during recovery from an action potential
  • there is increased excitability; smaller depolarization is required to evoke an AP and may even cause a train of APs
  • K accumulation in trasverse tubular system causes some depol. leading to spontaneous firing of the muscle after the end of nerve stimulation
20
Q

describe Duchenne muscular dystrophy

A
  • X-linked recessive disorder os muscles in males
  • cause: absence of muscle protein, dystrophin
21
Q

describe LMN syndrome

A
  • lower motor neurons
    • primary motor neurons of the spinal cord and brain stem that directly innervate skeletal muscles
    • signs:
      • muscle weakness (paresis)
      • flaccid paralysis
      • hyporeflexia
      • fasciculations
      • fibrilliations
  • upper motor neurons
    • neurons that originate in higher regions of the brain (cortex) and synapse on LMNs
22
Q

describe the expression of nAChRs in normal muscles

A
  • nAChRs scatter broadly but sparsely across embryonic muscle cells
  • in adult muscle fibers, normal nerve stimulation leads to aggregation of nAChRs under nerve endings
23
Q

explain the expression of nAChRs in denervated muscles

A
  • following denervation, nAChRs return to a supersensitive embryonic state with an adult state re-emerging with reinnervation

NB 3 NeuroPhys (14 decks)

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